For illustration of the background to the invention, suppose two parties “A” and “B” are engaged in a voice call with each other. Each party has a communication terminal, and the two terminals communicate with each other by means of a network composed of various equipment. Each communication terminal can sense audio from its user and transmit that audio to the other party, and can reproduce audio signals received from the other party.
During the voice call, party A might put party B on hold. In this context, the term “hold” may have a number of connotations, but typically it indicates that party A has set his terminal so as not to send to party B's terminal audio signals picked up from party A. Whilst party B is on hold, party A's terminal might generate a variety of alternative audio outputs to party B.    a) One option is for party A's terminal to transmit no audio signals to party B's terminal, in which case party B's terminal might produce no audio output whilst it is on hold.    b) A second option is for party A's terminal to generate a substitute audio stream and transmit it to party B's terminal in the same way as audio from party A would be transmitted. That substitute audio stream can then be played out by party B's terminal. Typically the substitute audio stream is music, comfort noise, a repeated message reassuring party B that he is on hold, or a series of beeps. Instead of being generated by terminal A, the substitute audio could be generated by a component in the network path between party A's terminal and party B's hardware; conveniently by a PBX (private branch exchange) local to party A.
A disadvantage of option a) is that whilst party B's terminal is reproducing no audio, party B might be concerned that the voice call has actually been disconnected. He might then terminate the call and have to re-establish communications with party A. A disadvantage of option b) is that there is no way for party A to know whether the audio that his terminal is generating is to party B's taste. Party B might dislike music that is being generated by party A's terminal, or might find other forms of substitute audio from party A annoying.
It would be desirable for a system or terminal to be able to improve the experience of a user whose terminal has been placed on hold without the need for cooperation from or adjustment of the rest of the system which resulted in them being placed on hold.
There is therefore a need for an improved mechanism for serving a party who is on hold.